I. Field of the Invention
The present invention relates to data communications. More particularly, the present invention relates to a novel and improved method and apparatus for resolving phase ambiguities in trellis coded modulated data.
II. Description of the Related Art
The field of data communications is concerned with raising the data throughput of a transmission system with a limited signal to noise ratio (SNR). The use of error correcting circuitry such as the Viterbi decoder allows system tradeoffs to be made with smaller SNRs or higher data rates to be used with the same bit error rate (BER). The decrease in the SNR needed is generally referred to as coding gain. Coding gain may be determined from simulated bit error performance curves. In a graph of simulated bit error performance curves the BER of uncoded and various code rate data is charted against E.sub.b /N.sub.o, where E.sub.b is the energy per bit and N.sub.o is the Gaussian White Noise energy per bit. The coding gain at any point along a bit error performance curve for a particular BER level is determined by subtracting the coded E.sub.b /N.sub.o from the uncoded E.sub.b /N.sub.o. In the paper "Viterbi Decoding for Satellite and Space Communication", by J. A. Heller and I. M. Jacobs, IEEE Transactions on Communication Technology, Vol. COM-19, pgs. 835-848, October 1971, extensive results of simulations on various decoder apparatus were reported.
The coding rate and constraint length are used to define the Viterbi decoder. The coding rate (m/n) corresponds to the number of coding symbols produced (n) for a given number of input bits (m). The coding rate of 1/2 has become one of the most popular rates, although other code rates are also generally used. One class of codes with m.noteq.1 are called punctures codes and are produced by discarding or erasing symbols from the rate 1/n code. The constraint length (K) is the length of the convolutional code used in the encoding of the data. A constraint length of K=7 is typical in convolutional coding schemes. The convolutional encoder can be thought of as an Finite Impulse Response (FIR) filter with binary coefficients and length K-1. This filter produces a symbol stream with 2.sup.K-1 possible states.
The basic principal of the Viterbi algorithm is to take a convolutionally encoded data stream that has been transmitted over a noisy channel and use the properties of the convolutional code to determine the transmitted bit stream. The Viterbi algorithm is a computationally efficient method of updating the conditional probabilities of the best state and the most probable bit sequence transmitted from the possible 2.sup.K-1 states. In order to compute this probability, all 2.sup.K-1 states for each bit must be computed. The resulting decision from each of these computations is stored as a single bit in a path memory.
A chainback operation, an inverse of the encoding operation, is performed in which the 2.sup.K-1 decision bits are used to select an output bit. After many states the most probable path will be selected with a high degree of certainty. The path memory depth must be sufficiently long to allow this probability to approach 1. For a rate 1/2 code, an exemplary path memory depth is about (5.multidot.K), or 35 states. For a rate 7/8 punctured code the optimal depth increases to 96 states.
Constraint lengths of K less than 5 are too small to provide any substantial coding gain, while systems with K greater than 7 are typically too complex to implement as a parallel architecture on a single VLSI device. As the constraint length increases, the number of interconnections in a fully parallel computation section increases as a function of (2.sup.K-1 .multidot.L), where L is the number of bits of precision in the state metric computations. Therefore, where K is greater than 7, serial computation devices are generally used which employ large external random access memories (RAMs).
In the paper "Channel Coding with Multilevel/Phase Signal" by G. Ungerboeck, IEEE Transactions on Information Theory, Vol. IT-28, pgs. 55-67, January 1982, a trellis coded modulation (TCM) was described. In Ungerboeck it was shown that within a given spectral bandwidth, it is possible to achieve an Asymptotic Coding Gain of up to 6 dB by employing a rate (k-1)/k convolutional code and doubling the signal set. Unfortunately for each modulation technique and for each bit rate, the maximal coding again is achieved by a different convolutional code. Further disclosed were the results of a search for all convolutional codes for several rates and modulation techniques, and the best codes presented.
In the paper "A Pragmatic Approach to Trellis-Coded Modulation" by A. J. Viterbi, J. K. Wolf, E. Zehavi and R. Padovani, IEEE Communications Magazine, pgs. 11-19, July 1990, a pragmatic approach to trellis coded modulation (PTCM) was disclosed. The underlying concept therein is that a somewhat lower coding gain is achievable by a PTCM based on the "industry standard" rate 1/2, K=7 convolutional code. Although a lower coding gain is realized, it is very close to the coding gain of Ungerboeck at BERs of interest.
Trellis coding is an attractive coding technique since it possesses an aspect which other coding techniques lack. The power of trellis coding lies in the fact that even though no apparent coding operation is performed on any other bit than the least significant bit of the input data, the decoder is able to provide error correction on all bits. Generally the use of TCM techniques to achieve efficient use of power-bandwidth resources has been limited to low speed applications in digital signal processor implementations. The use of PTCM techniques enable VLSI implementations of an encoder/decoder capable of operating at high rates. A decoder using PTCM techniques is capable of handling different modulation techniques, such as M-ary phase-shift keying (M-ary PSK) including Binary PSK (BPSK), Quadrature PSK (QPSK), 8-PSK, and 16-PSK.
In any phase modulation system, a stable reference is required for coherent demodulation at the receiver. However in a M-ary PSK system, where there is no absolute phase reference, phase ambiguities in the transmitted data may occur in the transmission channel which must be resolved for proper decoding of the received data. Such phase ambiguities, or phase shifts, in the received data can be on the order of 2.pi./M, 4.pi./M, . . . , (M-1)(2.pi./M). All phase ambiguities other than integer multiples 4.pi./M can be resolved by known techniques, typically monitoring the growth rate of the state metrics in the Viterbi decoder. However, such techniques do not enable resolution of the phase ambiguities that are integer multiples 4.pi./M. Therefore, the power of trellis coding has been unavailable to high speed data communication systems using M-ary PSK modulation.
It is therefore an object of the present invention to provide a novel method and system for resolving, in an M-ary PSK communication system, phase ambiguities of trellis coded modulated data that are integer multiples 4.pi./M.